1. Field of the Invention
The present invention relates to a borosilicate glass with a steep UV absorption limit or edge, which has an outstanding hydrolytic resistance and is sufficiently fusible with metals or alloys, to a method of making it and to a method of use of this borosilicate glass.
2. Description of the Related Art
Glasses with great hydrolytic resistance are already known. This type of glass is especially useful for glass-metal seals, which for example are useful in chemically corrosive environments, such as chemical plants or reactor structures. These glasses have a thermal expansion coefficient, α20/300, between 4.3 and 5.7*10−6/K. They are thus especially suitable for sealing with Fe—Co—Ni alloys, for example the so-called KOVAR alloys, and with molybdenum. The thermal expansion coefficient, α20/300, for tungsten amounts to from 3.4 to 4.3*10−6/K.
Glasses with strong UV-absorbing properties are known. For example those glasses find use in gas discharge lamps and often block UV radiation up to about 260 nm (layer thickness of 0.2 mm). This sort of gas discharge lamp produces a large fraction of UV light, which can damage neighboring components, as well as visible light. Especially those components, which contain polymers and plastic, are made brittle by this UV radiation during long-term usage, which can render the entire product unusable. For example, it has been shown that mercury produces an especially damaging emission line at 318 nm. It is the purpose of this type of lamp glass to prevent this especially damaging emission line from passing through it and to absorb it as completely as possible.
It has been shown that this sort of glass indeed has a significant absorption of visible light in a range under 1000 nm, which is disadvantageous from many applications. Also gas discharge tubes, such as fluorescent lights, which are used for liquid crystal displays (LCDs), especially of backside-illuminated displays, so-called backlight displays, have this disadvantage. Although this sort of fluorescent light has only very small dimensions and thus only has extremely thin lamp glass, a quality or grade loss occurs in spite of that, which has proven disadvantageous in qualitatively high-grade displays, such as electronic display units and computer screens, for example for laptops or mobile telephones.
Furthermore glasses in this sort of application should have a comparatively constant permeability and/or transmission, particularly for visible light up to a wavelength range of under 400 nm, especially under 380 nm, which then steeply drops.
Moreover it has been shown that this sort of fluorescent lamp glass has only a small hydrolytic resistance of class 3 according to ISO 719. This hydrolytic resistance is not sufficiently suitable for many products for processing this sort of glass and its use as lighting means.
Furthermore it is required that a glass especially for this sort of application must have certain definite physically properties, such as CTE, Tg, VA, which are required for sealing with metals, such as tungsten and molybdenum, and metal alloys, such as KOVAR. For example, a CTE of 4.3 to 5.5*10−6/K (30-380° C.) is required for KOVAR, a CTE of 4.4 to 5.1*10−6/K (30-380° C.) is required for molybdenum and a CTE of 3.4 to 4.3*10−6/K (30-380° C.) is required for tungsten. The glass temperature Tg is preferably from 470° C. to 540° C. Attempts are made to provide glasses of this sort with a hydrolytic resistance of at least class 2, preferably class 1 according to ISO 719.
Zirconium oxide-containing and lithium oxide-containing borosilicate glasses with high chemical resistance, which have a high hydrolytic resistance, a high acid resistance and a high alkali resistance and which are especially suitable for laboratory applications, for chemical plants and pharmaceutical packaging as well as mantel glass fibers, are known from DE-A 198 42 942. Furthermore this sort of glass is especially suitable for glass-metal seals.
The borosilicate glass for discharge lamps described in JP-A 8-12369 contains total amounts of from 0.03 to 3 percent by weight of at least two of four ingredients: V2O5, Fe2O3, TiO2 and CeO2 for UV blocking. High transmission and high solarization resistances are not adjustable with these ingredients with high individual ingredient amounts and their combinations. Many of these glasses have a noticeable discoloration during sealing or melting.
U.S. Pat. No. 5,747,399 discloses a glass for miniaturized fluorescent lamps, which have solarization stability and UV-impermeability because of TiO2 and/or PbO and/or Sb2O3. However amount of TiO2, especially high amounts, lead to coloration of the glass. Also PbO should be avoided due to environmental problems.
Furthermore fluorescent lamp glasses are known from U.S. Pat. No. 5,747,399 for the above-described applications, which absorb UV-radiation in the desired amounts. However it has been shown that this sort of glass exhibits a strong solarization and a strong discoloration in the visible range.
Moreover a lamp glass for a fluorescent lamp is known from JP-A 2002 293 571, which is especially suitable for illumination of liquid crystal displays.
A zirconium oxide-containing and lithium oxide-containing borosilicate glass of high resistance is known from DE-A 198 42 942, which is especially suitable for use as sealing glass for sealing with Fe—Co—Ni alloys. This sort of glass can also contained colored ingredients, such as Fe2O3, Cr2O3, CoO and TiO2.
In U.S. Pat. No. 4,565,791 A glass is described for ophthalmologic applications, which has a special index of refraction and Abbé number, and a suitable density for that application. This sort of glass has a UV absorption limit or edge between 310 nm and 335 nm and contains TiO2 as UV absorber. For manufacture of this glass this reference expressly teaches that refining with chlorine is required, since refining with As2O3 and with Sb2O3 is not sufficient. Finally the reference teaches that although these glasses are extremely thin, a combination of Fe2O3 and TiO2 leads to a discoloration of the glass so that quartz raw material should be used exclusively with an iron content of less than 100 ppm.
It has also been shown that this sort of glass has the above-described disadvantages of the state of the art, such as strong solarization, discoloration and absorption in the visible wavelength range.